Authors have different ways to approach that first draft. Some work from a brief outline of bulletpoints, others from a detailed outline of plots points and dialogue. Still others rely on a freeform, improvisational outline with a highlight or two and no ending in sight.
I prefer the first of these. I know my beginning, my ending, and key points in the middle, but always feel free to write what comes to mind in the course of the story. Usually that works for me. It's like improv--structure and direction, but no defined product.
I bring this up now because I recently hit a wall in writing that way. Was working on a short story, complete with known opening and closing, but letting it flow in the middle. Couldn't figure out why it wasn't working for me. I liked some of the scenes, the way they played out, the way the flowed into each other, although I was having trouble placing the key scene, the one that explains the title. I returned to my outline to see exactly where I should put it and then realized that not only had I failed to place that scene, I completely forgot to add the scene detailing the main character's motivation. Well, I quickly figured out where to put the title scene, but recognized that I had to completely rewrite the motivation scene. I had hinted at it, had started in the wrong place and exited too early, and never developed it fully.
That's why we have rewrites.
It's also why we have outlines.
It's OK to vary from an outline, sometimes sharply. It's even OK to revamp them completely if they're not working. But if you have one, at least look at it once in awhile. I never even had that nagging feeling that I'd missed something--I'd just missed something! And made a lot more work for myself in the process.
After more than two dozen short stories, a novel, 15 or so children stories, and more than a thousand print and online pieces, you'd think I'd know better. Well, here's hoping I don't make that mistake again...
On the other hand...this post had no outline or ending. Stream-of-consciousness all the way. See paragraph one, style three. Sometimes I should just kick myself. Then again, always feel free to vary your routine. Experiment a bit. Or a lot. Eventually something will work...